The present invention refers to a method for initiating the automatic regulation of the operating current of a current source in a power supply unit upon closing the operating load circuit connected to said current source. Further, the invention refers to a power supply unit having a regulating circuitry, particularly for supplying power to a plasma burner unit.
Upon closing the load circuit of a power supply unit in which the operational current is automatically regulated, the possibility exists that current peaks occur due to the dynamic behaviour of the regulation circuitry included in the power supply unit, due to the behaviour of the switching means, serving for closing the load circuit and/or due to the behaviour of the apparatus connected to the load circuit of the power supply unit. These current peaks may lead to problems in the behaviour of the apparatus connected to the power supply means or even may result in damage of electric or electronic components included in this apparatus.
If the regulation circuitry is well designed, it is often possible to keep such current peaks occurring during the initial operating phase within acceptable limits or even to avoid them more or less. However, in many cases, it is very difficult to approach this goal with known measures. For example, one is forced to use a regulation method in which the control signal for the regulation is not continuously affecting the regulation circuitry, but only intermittently.
One example for such a difficult situation is the regulation of electrically controllable diodes, particularly thyristors, by means of a phase shift control mechanism. It is well known in the art that in an electrically controllable diode of the kind mentioned above a current flows beginning with the moment of the ignition of the electrically controllable value up to the zero crossing potential of the alternative current or, in the case of a multi-phase rectifier assembly, up to the transition of the current to another electrically controllable diode. There is no possibility to momentarily influence the current through the diode as far as its amount is concerned. Only after the next ignition of the electrically controllable diode is it possible to influence the amount of the current flowing in the load circuit by changing the ignition timing. In the meantime, the amount of current flow is not controllable and can get very large. In other words, due to the time lag of the regulation process severe transients can occur which cannot be avoided by a regulation process of the kind described hereinabove.
The reason for the above mentioned problems may be seen in the fact that usually the current regulation starts just at the beginning of the flow of current in the load circuit with the result that the current regulator must handle quite a large difference between desired or preset current and effective current. Consequently, it should be possible to solve the problem if the aforementioned difference between desired or preset value of the current and effective value of the current is as small as possible in the moment when the current regulation begins to operate, i.e. if the effective value of the current flowing in the load circuit has already reached the desired or preset value at the moment when the operation of the current regulator begins. In other words, the current flowing in the load circuit should be increased to the desired or preset value just at the beginning of the operation, before the current regulator starts to influence the current value. This possibility exists on condition that the current flowing at the moment of closing the load circuit can be effectively limited by other measures up to the moment when the current regulator starts to operate.